Optical device of waveguide type and its production method

ABSTRACT

Input ports ( 103   a,    103   b ) formed from fundamental mode waveguides are provided at one end of a multimode waveguide ( 104 ). Further, an output port ( 105 ) formed from a fundamental mode waveguide is provided at the other end of the multimode waveguide ( 104 ). The multimode waveguide ( 104 ) has a width wider than those of the input ports ( 103   a,    103   b ) and the output port ( 105 ), and provides modes including multimode to the waveguide. The multimode waveguide ( 104 ) is embedded with a buried layer ( 200 ). Both of the end faces of the multimode waveguide ( 104 ) are made to be planes equivalent to a (100) plane or planes inclined from these planes. In a case of inclined planes, the planes are made to be planes inclined to a direction that the waveguide region spreads toward a stacked direction of the semiconductor layers.

This application claims priority from PCT Application No.PCT/JP2004/012468 filed Aug. 30, 2004 and from Japanese Application No.2003-303998 filed Aug. 28, 2003, which applications are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a waveguide optical device and amanufacturing method thereof.

BACKGROUND ART

In recent years, in proportion to increases in the volume ofcommunication information, communication in a wavelength divisionmultiplexing transmission system in which a plurality of optical signalshaving different wavelengths are multiplexed to one optical fiber hasbeen broadly utilized. In this method, an optical integrated circuitstructured from a coupler which couples optical signals having aplurality of wavelengths, a branching filter which branches a pluralityof optical signals in one optical fiber into different ports, or thelike, in addition to a light emitting element and a light receivingelement, is used.

As such a coupler, a branching filter and the like which structure anoptical integrated circuit, various types of those can be used. Awaveguide semiconductor device thereamong is suitable for anaccumulation with other waveguide devices and passive waveguides, and ispreferably used.

Further, as an example of a waveguide device, there are cases in whichan MMI (Multi Mode Interference) structure is used (Patent Document 1).In an MMI type coupler/branching filter, a coupling/branching functionof light waves is realized by using a multimode waveguide in whichhigher mode propagating is possible, and by utilizing interference amongthe respective modes in the waveguide. By using an MMI type structure,it is possible to reduce optical loss, and to improve the stability inmanufacture.

FIG. 21 is a diagram showing a schematic structure of a waveguideportion of a conventional waveguide coupler. A multimode waveguide 101having a mesa shape which is formed such that a core layer and a guidelayer are stacked in this order is formed on a substrate 100. Themultimode waveguide 101 is structured so as to be embedded with a buriedlayer 200. This coupler has input ports 103 a and 103 b formed fromfundamental mode waveguides at one end of the multimode waveguide 101,and has an output port 105 formed from a fundamental mode waveguide atthe other end of the multimode waveguide 101. The multimode waveguide101 has a width wider than the input ports 103 a and 103 b, and theoutput port 105, and provides modes including multimode with respect tothe waveguides.

Single mode lights respectively having peculiar wavelengths are incidentinto the input ports 103 a and 103 b, and these are guided into themultimode waveguide 101. The incident lights advance inside themultimode waveguide 101 while varying interference patterns inaccordance with an advanced position, and are emitted from the outputport 105 on the right side in the drawing.

In this coupler, the optical output face is a (110) plane, and a lightis guided in the <110> direction. The side faces of the multimodewaveguide 101 are side faces parallel to the <110> direction, and theend face at the optical output side is formed from the (110) plane, andthe end face at the optical incident side is formed from the (−1-10)plane. [Patent Document 1] U.S. Pat. No. 5,640,474.

DISCLOSURE OF THE INVENTION

However, with respect to a shape of such a conventional waveguideoptical device, when the periphery of the core layer is embedded with asemiconductor, there has been frequent that an optical loss is caused,and a variation in the element performance is brought about. An objectof the present invention is to solve such a problem, and to provide ahigh-efficiency waveguide optical device with less optical loss. Theinventors have considered the cause by which an optical loss is causedas described above. As a result, the inventors have found the cause thatan InP buried layer disposed at the periphery of the waveguide opticaldevice brings about abnormal growth at the mesa side face of thewaveguide optical device.

FIG. 21 is a schematic diagram of the conventional MMI type couplerdescribed above. The mesa 101 structuring waveguides is formed on thesubstrate 100 formed from InP. The mesa 101 is formed from a multimodewaveguide at the central portion, and fundamental mode waveguidesconnected to the both ends. In an coupler using an InGaAsPsemiconductor, usually, as illustrated, a structure in which the <110>direction is a waveguide direction, and side faces parallel to and sidefaces perpendicular to this direction are provided, and the (110) planeis an optical output face is used. However, in this case, it has becomeapparent by the consideration of the inventors that abnormal growth ofthe semiconductor layer is easily brought about at {110}planes which arethe side faces of the multimode waveguide at the time of forming the InPburied layer at the periphery of the mesa.

FIG. 8 is a diagram showing a state at the midterm stage ofmanufacturing the coupler of FIG. 21, and a diagram for explanation of astate of abnormal growth of the InP buried layer. In the drawing, a corelayer 108 and an upper guide layer 110 are stacked on the substrate 100,and a mask 112 is provided above those. A semiconductor layer isembedded to grow at the periphery of the mesa by using the mask 112, andan InP layer 115 is formed. As illustrated, the InP layer 115 hasabnormally grown so as to cover the mask 112 to become a shape greatlybuilt up as compared with the mesa. When such abnormal growth of asemiconductor layer is brought about, optical loss and reflection arefrequently brought about.

The inventors have reached the present invention as a result of theconsideration on the assumption that optical loss and reflection can besuppressed by suppressing such abnormal growth.

According to this invention, there is provided a waveguide opticaldevice including a waveguide in which a core layer and a guide layerwhich are formed from semiconductors having zinc blend crystalstructures are stacked in this order,

wherein the waveguide has fundamental mode waveguides providing afundamental mode to a waveguide light, and a multimode waveguide whichhas a width wider than the fundamental mode waveguides, and whichprovides modes including multimode to a waveguide light, and

the multimode waveguide includes side faces structured from planesequivalent to a (100) plane of the semiconductor, or planes having anangle of inclination to a stacked direction of the core layer and theguide layer, and/or an off angle less than or equal to 7° in an in-planedirection of the core layer and the guide layer.

Further, according to this invention, there is a method of manufacturinga waveguide optical device, including:

forming a stacked layer including a core layer and a guide layer whichare formed from semiconductors having zinc blend crystal structures;

forming a mesa portion including fundamental mode waveguides and amultimode waveguide by selectively removing the guide layer and the corelayer; and

forming a semiconductor layer so as to embed the periphery of the mesaportion,

wherein side faces of the multimode waveguide form the mesa portion soas to be in a form including

planes equivalent to a (100) plane of the semiconductor, or

planes having an angle of inclination to a stacked direction of the corelayer and the guide layer, and/or an off angle less than or equal to 7°in an in-plane direction of the core layer and the guide layer.

According to the present invention, at least a part of the side faces ofthe multimode waveguide are formed from planes equivalent to the (100)plane, or planes having an angle of inclination to a stacked directionof the core layer and the guide layer, and/or an off angle less than orequal to 7° in the in-plane direction of the core layer and the guidelayer. The planes equivalent to the (100) plane mean the (100) plane, a(010) plane, a (−100) plane, and a (0-10) plane. Because the planes usedin the present invention have the characteristic that abnormal growth ofa semiconductor layer is markedly suppressed, building-up of the buriedlayer can be stably reduced. In accordance therewith, optical loss atthe end face of the multimode waveguide can be effectively reduced. Sucha characteristic as described above will be described later in theembodiments. Note that the above-described angle of inclination ispreferably made to be less than or equal to 45°. In this way, abnormalgrowth of a semiconductor layer can be certainly suppressed.

In the present invention, all the end faces of the multimode waveguideare preferably structured from the above-described specific planes.However, a part of the end faces may be structured from planesperpendicular to an optical waveguide direction.

The shapes of the end faces of the core layer and the guide layer at theside faces of the multimode waveguide may be made to be in variousaspects.

For example, the end faces of the guide layer at the above-describedside faces may be planes having an off angle less than or equal to 5° tothe stacked direction of the core layer and the guide layer. When thecore layer and the guide layer are stacked in the <001> direction, theplanes are planes substantially perpendicular to the (001) plane. Inthis way, an extent of building-up of the semiconductor layer isuniformed, and it is possible to reduce variations among the elements.

Further, the end faces of the core layer at the above-described sidefaces may be planes having an off angle less than or equal to 5° to thestacked direction of the core layer and the guide layer. When the corelayer and the guide layer are stacked in the <001> direction, the planesare planes substantially perpendicular to the (001) plane. In this way,it is possible to reduce variations of building-up of the semiconductorlayer among the elements.

Moreover, the end faces of the core layer may be withdrawn from the endfaces of the guide layer at the side faces. In this way, because thecore layer is formed so as to be withdrawn from the guide layer, a givenamount of semiconductor materials is contained in this withdrawnportion, and building-up of the semiconductor layer at the end faces ofthe multimode waveguide can be further reduced.

As a material composing the core layer, for example,

there is shown In_(x)Ga_(1-x)As_(y)P_(1-y) (x and y are numbers greaterthan or equal to 0 and less than or equal to 1).

The multimode waveguide in the present invention is a multimodeinterference type waveguide, and the input or the output, or both ofthose may be formed from a plurality of ports.

In the method of manufacturing the present invention, the semiconductorlayer embedding the periphery of the mesa may be formed by epitaxialgrowth using a growth gas including a halogen gas. In this way, anextent of building-up of the semiconductor layer and the variationthereof can be effectively reduced.

The waveguide optical device in the present invention may be structuredso as to have a plurality of input ports or a plurality of output ports,and to have a branching function or a coupling function. Moreover, itmay be an optical device such as an optical amplifier by having astructure in which the core layer includes a gain layer (a layer inwhich an optical gain can be obtained). Moreover, it may be alight-receiving device by having a structure in which the core layerincludes a light-receiving layer.

Further, the present invention includes the following aspects as well.

(i) A waveguide optical device including a waveguide in which a corelayer and a guide layer which are formed from semiconductors having zincblend crystal structures are stacked in this order, wherein

the waveguide includes fundamental mode waveguides providing afundamental mode to a waveguide light, and a multimode waveguide whichhas a width wider than the fundamental mode waveguides, and whichprovides modes including multimode to a waveguide light, and

the multimode waveguide includes side faces structured from planesequivalent to the (100) plane, or planes which are inclined to thevertical line of the substrate face with respect to those planes, andwhich have an off angle less than or equal to 7° in the in-planedirection of the substrate face.

(ii) A method of manufacturing a waveguide optical device, comprising:

forming a stacked layer including a core layer and a guide layer on asubstrate;

forming a mesa portion including fundamental mode waveguides and amultimode waveguide by selectively removing the guide layer and the corelayer; and

forming a semiconductor layer so as to embed the periphery of the mesaportion,

wherein end faces of the multimode waveguide form the mesa portion so asto include planes equivalent to a (100) plane, or planes which areinclined to the vertical line of the substrate face with respect tothose planes, and which have an off angle less than or equal to 7° inthe in-plane direction of the substrate face.

As described above, in accordance with the present invention, becausethe side faces of the embedded multimode waveguide are structured fromspecific planes, a high-efficiency waveguide optical device with lessoptical loss is stably provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The object described above, and the other objects, features, andadvantages will become further apparent from the preferred embodimentswhich will be described, and the accompanying following drawings.

FIG. 1 is a diagram showing a structure of a coupler relating to anembodiment.

FIG. 2 is a diagram showing a structure of the coupler relating to theembodiment.

FIG. 3 is a diagram showing a layered structure of the coupler relatingto the embodiment.

FIG. 4 is a diagram showing a layered structure of the coupler relatingto the embodiment.

FIG. 5 is a diagram for explanation of a method of manufacturing thecoupler shown in FIG. 1.

FIG. 6 is a diagram for explanation of a method of manufacturing thecoupler shown in FIG. 1.

FIG. 7 is a diagram for explanation of a method of manufacturing thecoupler shown in FIG. 1.

FIG. 8 is a diagram showing a state of abnormal growth of asemiconductor layer in a conventional coupler.

FIG. 9 is a diagram showing the outline of an optical coupling circuitrelating to an embodiment.

FIG. 10 is a diagram showing a structure of a branching filter relatingto the embodiment.

FIG. 11 is a diagram showing a structure of an optical receiver relatingto the embodiment.

FIG. 12 is a diagram showing a layered structure of the optical receiverrelating to the embodiment.

FIG. 13 is a diagram showing a layered structure of the optical receiverrelating to the embodiment.

FIG. 14 is a diagram for explanation of a method of manufacturing thecoupler shown in FIG. 11.

FIG. 15 is a diagram for explanation of a method of manufacturing thecoupler shown in FIG. 11.

FIG. 16 is a diagram for explanation of a method of manufacturing thecoupler shown in FIG. 11.

FIG. 17 is a diagram showing a structure of an optical amplifierrelating to an embodiment.

FIG. 18 is a diagram showing a structure of the optical amplifierrelating to the embodiment.

FIG. 19 is a diagram showing a structure of the optical amplifierevaluated in an example.

FIG. 20 is a diagram for explanation of variations in a growth rate of asemiconductor layer on a bonded surface.

FIG. 21 is a diagram showing a structure of a coupler relating to arelated art.

FIG. 22 is a diagram showing a structure of a coupler relating to anembodiment.

FIG. 23 is a diagram showing a structure of an amplifier relating to anembodiment.

FIG. 24 is a diagram showing a structure of the amplifier relating tothe embodiment.

FIG. 25 is a diagram showing a state after the growth of a mask in theprocess of obtaining the structure of FIG. 19.

FIG. 26 is a diagram for explanation of a method of manufacturing thecoupler shown in FIG. 1.

FIG. 27 is a diagram showing a structure of an exemplary embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

All the following respective embodiments are the examples ofsemiconductor optical devices using InP semiconductors having zinc blendcrystal structures. Namely, an InGaAsP semiconductor is used as a corelayer, and InP is used as a buried layer. Hereinafter, the detailsthereof will be described. Note that, in the following descriptions,same members are denoted by the same reference numerals, andexplanations thereof will not be repeated.

First Embodiment

FIG. 1 is a pattern diagram showing a waveguide structure of a couplerrelating to the present embodiment. FIG. 2 is a top view of the deviceshown in FIG. 1. As shown in FIG. 1, the coupler relating to the presentembodiment has a structure in which a waveguide is provided on asubstrate 100. Input ports 103 a and 103 b formed from fundamental modewaveguides are provided at one end of a multimode waveguide 104.Further, an output port 105 formed from a fundamental mode waveguide isprovided at the other end of the multimode waveguide 104. The multimodewaveguide 104 has a width wider than the input ports 103 a and 103 b,and the output port 105, and provides modes including multimode to thewaveguide. The multimode waveguide 104 is embedded with a buried layer200. Note that, as the substrate 100, InP with a (001) plane serving asa principal surface is used in the present embodiment.

The multimode waveguide 104 and the input ports 103 a and 103 b formedfrom fundamental mode waveguides have a structure in which, as will bedescribed later, a core layer and a guide layer which are formed fromsemiconductors with zinc blend crystal structures are stacked in thisorder.

In FIG. 2, all of the end faces of the multimode waveguide 104 shown bya1 to a4 are planes equivalent to the (100) plane (hereinafter,appropriately called {100} planes), or planes inclined from thoseplanes. In a case of inclined planes, those are preferably made to beplanes having (a) an angle of inclination to the stacked direction ofthe core layer and the guide layer, and/or (b) an off angle less than orequal to 7° in the in-plane direction of the core layer and the guidelayer. For example, those may be made to be planes which are inclined tothe vertical line of the substrate face, and which have an off angleless than or equal to 7° in the in-plane direction of the substrateface. The inclination to the stacked direction is preferably embodied soas to be inclined to a direction that the waveguide region spreadstoward the stacked direction of the semiconductor layer, and the angleof inclination is preferably made to be less than or equal to 45°. Inthis case, the end faces are not necessarily single flat surfaces, butmay be structured from a plurality of flat surfaces, or may be curvedsurfaces formed by wet etching or the like.

In the present embodiment, as a1 to a4, the following planes, or planesinclined within a predetermined range with respect to those planes areused.

-   a1: (0-10) plane-   a2: (100) plane-   a3: (010) plane-   a4: (−100) plane

In FIG. 2, b is an optical output face, the (110) plane of thesemiconductor layer structuring the waveguide is exposed. The faces c1and c2 are a (1-10) plane and a (−110) plane, respectively.

FIG. 3 is an A-A′ cross-sectional view of FIG. 2. As illustrated, a corelayer 108 and an upper guide layer 110 are stacked on the substrate 100,and an InP layer 115 whose refractive index is lower than that of theupper guide layer 110 is formed at the both sides thereof. Here, thecore layer 108 is composed from InGaAsP, and the upper guide layer 110is composed from InP. The core layer 108 and the upper guide layer 110are stacked in the <001> direction.

FIG. 4 is a B-B′ cross-sectional view of FIG. 2. It is a stackedstructure as illustrated in the vicinity of the side walls of themultimode waveguide 104. That is, InP layer 115 is embedded in the sidewalls of the mesa in which the core layer 108 and the upper guide layer110 are stacked on the substrate 100. In a related art, the side wallsof the mesa at the MMI region are structured from the (110) planes whichare the same as the wall interface plane, and when the planes areembedded, there are many cases in which the InP layer 115 brings aboutabnormal growth as shown in FIG. 8, and the InP layer 115 is formed soas to cover the upper guide layer 110. In such a shape, the buriedstructure is not preferably formed, and an extent of optical loss due toreflective spots being generated at the side faces is made greater.

In the present embodiment, because the side walls of the mesa are madeto be {100} planes or planes inclined to the <100> direction withrespect to those planes, namely, the stacked direction of the core layerand the guide layer, and to be planes having an off angle less than orequal to 7° in the in-plane direction of the substrate face, abnormalgrowth of such a semiconductor layer is suppressed, and a stackedstructure in a shape, as the design as shown in FIG. 4 is, can beobtained. As shown in the drawing, the top face of the InP layer 115 ispositioned so as to be slightly higher than the top face of the upperguide layer 110, and is formed as a flat plane. Note that, in thedrawing, the side wall of the mesa which is formed from the core layer108 and the upper guide layer 110 is perpendicular to the substrate.However, the side wall may be structured so as to be inclined to themesa stacked direction.

Next, a method of manufacturing a coupler shown in FIG. 1 to FIG. 4 willbe described with reference to FIG. 5 to FIG. 7. First, as shown in FIG.5, the core layer 108 and the upper guide layer 110 are formed on thesubstrate 100. Those can be formed by, for example, an MOVPE method orthe like. With respect to the layer thicknesses of the respectivelayers, for example, respectively, the lower core layer 108 can be setto about 100 nm, and the upper guide layer 110 can be set to about 600nm.

Next, as shown in FIG. 6, a mask 112 is provided on the upper guidelayer 110 by utilizing a photolithography technique and wet etching. Themask 112 is formed from, for example, oxide silicon or the like.

Next, the upper guide layer 110 and the core layer 108 are selectivelyetched to be a state of FIG. 6 by a reactive ion etching method. As anetching gas, a mixed gas including chlorine can be used. At this time, apart of the substrate 100 is over-etched to be in a form that thesurface of the substrate 100 is dug down. Because all the respectivelayers forming the mesa are processed by dry etching, the end faces ofthe respective layers (the core layer and the guide layer) at the sidefaces of the multimode waveguide are made to be planes substantiallyperpendicular to the substrate. Namely, those are made to be planeshaving an off angle less than or equal to 5° to the stacked direction ofthe core layer and the guide layer. In accordance therewith, an extentof building-up of the semiconductor layer can be uniformed.

Next, as shown in FIG. 7, the InP layer 115 is made to grow at the bothsides of the mesa including the core layer 108. As the growth method, anMOVPE method can be used. Here, the above-described semiconductor layermay be formed by epitaxial growth using a growth gas including a halogengas. In this way, abnormal growth can be more effectively suppressed.

Due to the mask 112 being removed from the sate of FIG. 7 by bufferedhydrofluoric acid or the like, and next, due to the InP layer 118 beingformed, the coupler having the structure shown in FIG. 1 to FIG. 4 isformed.

With respect to the coupler relating to the present embodiment, becausethe end faces are structured from planes equivalent to the (100) plane,the buried layer can be suppressed from being an abnormal shape, and itis possible to effectively reduce optical loss.

Further, the output side end faces of the multimode waveguide 104 havean inclination of 45° to the optical waveguide direction. The refractiveindex of the semiconductor material structuring the multimode waveguide104 and the refractive index of the semiconductor material embedded intothe periphery thereof are different from one another to a slight extent,and are values close to one another.

Therefore, the above-described angle of 45° corresponds to a so-calledBrewster's angle. Accordingly, a light which has directly advanced inthe MMI region, and which has not been coupled with the output port istotally transmitted at the end face at an angle of 45° to the opticalwaveguide direction. In accordance therewith, the effects by returnlight is reduced, which leads to a reduction of return light to theincident side.

Moreover, in the present embodiment, the end face of the multimodewaveguide 104 at the input port side is made to be a circular shape. Inaccordance therewith, three-dimensional growth due to a raw gas beingexcessively supplied to between the input ports at the time of carryingout embedding growth at the periphery of the multimode waveguide 104 byusing the mask can be suppressed. Note that the part of the circularshape may be a rectangular shape as in FIG. 22.

Second Embodiment

In the first embodiment, at the step of FIG. 6, the mesa shape is formedby selectively etching both of the upper guide layer 110 and the corelayer 108 by dry etching. In contrast thereto, in the presentembodiment, after the upper guide layer 110 is selectively etched by dryetching, the core layer 108 is selectively wet-etched by using anetchant including sulfuric acid with the mask 112 being maintained. Inthis way, it can be a form that the end face of the core layer 108 iswithdrawn from the end face of the upper guide layer 110, as shown, forexample in FIG. 26. Because a given amount of semiconductor materials iscontained in this withdrawn portion, and building-up of thesemiconductor layer at the end faces of the multimode waveguide can befurther reduced. Note that the end faces of the core layer 108 and theupper guide layer 110 may be made to be planes inclined from the {100}plane. The inclined planes may be planes having (a) an angle ofinclination to the stacked direction of the core layer and the guidelayer to (100) planes, and/or (b) an off angle less than or equal to 7°(see e.g., FIG. 27) in-plane direction of the core layer and the guidelayer to (100) planes.

Third Embodiment

FIG. 9 is an example of an optical coupling circuit using a couplerrelating to the present invention. This optical coupling circuit has astructure in which DFB light sources 170 and an MMI region 172 arecoupled with each other. An outgoing light from the MMI region 172 canbe guided into, for example, a semiconductor amplifier. The MMI region172 can be made to have a semiconductor stacked structure described inthe first and second embodiments.

The lights emitted from the DFB light sources 170 are guided into theMMI region 172. In the MMI region 172, the multimode lights interferewith one another, and the light is emitted from a light output unit 174.In this optical coupling circuit, the output side end faces of the MMIregion 172 are structured from a (100) plane and a (010) planeequivalent thereto. Because the side faces of the MMI region 172 arestructured from such planes, the following effects can be obtained.First, lights which have not been coupled at the output port reach the(100) plane and the (010) plane, but do not become reflective pointsbecause abnormal growth is not brought about at this region. Further,the above-described side faces have an angle of 45° to the waveguidedirection. However, because this angle is a Brewster's angle, the lightis totally transmitted. Therefore, it leads to a reduction of returnlight to the DFB light sources 170, which enables the stable operationof laser.

The example in which the present invention is applied to a coupler hasbeen described above. However, the present invention can be applied tovarious optical devices except a coupler. Hereinafter, such exampleswill be described. Note that, in the following examples, the multimodewaveguide and the semiconductor buried layer at the periphery thereofcan be manufactured by a method which is the same as those described inthe first and second embodiments.

Fourth Embodiment

FIG. 10 is a diagram showing a structure of a mesa shape of a branchingfilter relating to the present embodiment. An incident light is guidedfrom a port 1 to an MMI region 136. Lights branched in the MMI region136 are emitted from a port 2 and a port 3. In this branching filter,the following planes are used.

-   a1: (0-10) plane-   a2: (100) plane-   a3: (010) plane-   a4: (−100) plane    The respective ports are connected so as to be substantially    perpendicular to those planes. Due to such a structure being used,    when the mesa in the shape is embedded, abnormal growth is not    brought about on the above-described planes, and therefore, a    branching filter with less reflection and optical loss, which is    excellent in the stability of performance can be obtained.

Fifth Embodiment

FIG. 11 is a top view showing a structure of an optical receiverrelating to a present embodiment. This optical receiver is structuredsuch that a buried layer 126 is formed at the periphery of the opticalreceiver 125. Four side faces of the optical receiver 125 arerespectively structured from a (100) plane and planes equivalentthereto. These planes are formed by an etching process for forming amesa. However, these planes may be formed by only dry etching, and canbe formed by carrying out wet etching after carrying out dry etching.

In this optical receiver, a light is guided from a stripe shapedwaveguide 124 shown on the left side in the drawing to the opticalreceiver 125. The waveguide direction is the <110> plane. The side facesof the optical receiver are formed so as to have an angle of 45° to thiswaveguide direction. Therefore, the side faces are formed at anBrewster's angle to a waveguide light, and an attempt can be made toreduce return light to the system side. Further, as already described,by selecting these planes, abnormal growth of the semiconductor at theside faces of the mesa can be effectively suppressed, which results in areduction of reflective points.

FIG. 12 is an A-A′ cross-sectional view of FIG. 11. As illustrated, withrespect to the optical receiver relating to the present embodiment, alower guide layer 106, the core layer 108, and the upper guide layer 110are stacked on the substrate 100, and the both sides are embed with anFe-InP layer 190. Moreover, the optical receiver is structured such thatthe p-InP layer 118 and the p-InGaAs 120 are stacked on those. The corelayer 108 is composed from InGaAsP or InGaAs. The core layer 108functions as an optical receiving layer.

Note that the stacked direction of the lower guide layer 106, the corelayer 108, and the upper guide layer 110 is the <001> direction. Notethat, in the present embodiment, a convex portion is formed on thesubstrate 100. However, the substrate 100 may be a flat shape withoutany convex portion.

In FIG. 12 and FIG. 13, an Fe—InP layer is formed so as to contact withthe mesa. In the related art, there have been many cases in which theFe—InP layer has brought about abnormal growth, and the Fe—InP layer hasbeen formed so as to cover the upper guide layer 110. In such a shape,current stenosis structure is not preferably formed, which makes currentleakage large. Further, optical loss due to reflective points beinggenerated at the side faces, and return light to the system side due tothe reflection of light become problematic.

In contrast thereto, because the side walls of the mesa are made to be{100} planes or planes inclined to the <001> direction with respect tothese planes, namely, to the stacked direction of the core layer and theguide layer, and to be planes having an off angle less than or equal to7° in the in-plane direction of the substrate face, abnormal growth ofsuch a semiconductor layer is suppressed, and a stacked structure in ashape as the design as shown in FIG. 11 to FIG. 13 is, can be obtained.The top portion of the buried layer Fe—InP layer 190 is formed as a flatplane. Note that, in the drawing, the side wall of the mesa which isformed from the lower guide layer 106, the core layer 108, and the upperguide layer 110 is made perpendicular to the substrate. However, theside wall may be structured so as to be inclined to the mesa stackeddirection.

Next, a method of manufacturing the optical receiver relating to thepresent embodiment will be described with reference to the drawings.First, as shown in FIG. 14, the lower guide layer 106 formed from ann-type semiconductor, the core layer 108 formed from a non-doped layer,and the upper guide layer 110 formed from a p-type semiconductor areformed on the substrate 100. These can be formed by, for example, anMOVPE method or the like. With respect to the layer thicknesses of therespective layers, for example, respectively, the lower guide layer 106can be set to about 100 nm, the lower core layer 108 can be set to about100 nm, and the upper guide layer 110 can be set to about 600 nm.

Next, the mask 112 is provided on the upper guide layer 110 by utilizinga photolithography technique and wet etching. The mask 112 is formedfrom, for example, oxide silicon or the like. Next, the upper guidelayer 110, the core layer 108, and the lower guide layer 106 areselectively etched by a reactive ion etching method, and the state shownin FIG. 15 is obtained. As an etching gas, a mixed gas includingchlorine can be used. At this time, a part of the substrate 100 isetched, and a concave portion is formed on the surface of the substrate100. Because all the respective layers forming the mesa are processed bydry etching, the end faces of the respective layers at the side faces ofthe multimode waveguide are made to be planes substantiallyperpendicular to the substrate. Namely, the end faces are made to beplanes having an off angle less than or equal to 5° to the stackeddirection of the core layer and the guide layer, as shown, for example,in FIG. 26. In accordance therewith, an extent of building-up of thesemiconductor layer can be uniformed.

Thereafter, the buried structure shown in FIG. 16 is obtained byembedding the both sides of the mesa including the upper guide layer110, the core layer 108, and the lower guide layer 106 with the Fe—InP115.

In accordance with the optical receiver relating to the presentembodiment, the current stenosis structure at the both sides of the mesais stable, and the light receiving device characteristics such as darkcurrent characteristic or the like are favorable.

Sixth Embodiment

FIG. 17 is a pattern diagram showing a waveguide structure of an opticalamplifier relating to the present embodiment. FIG. 18 is a top view ofthe device shown in FIG. 17. As shown in FIG. 17, the optical amplifierrelating to the present embodiment has an input port 103 formed from afundamental mode waveguide at one end of the multimode waveguide 104.Further, an output port 105 formed from a fundamental mode waveguide atthe other end of the multimode waveguide 104. The multimode waveguide104 has a width wider than the input port 103 and the output port 105,and provides modes including multimode to the waveguide.

All the end faces of the multimode waveguide 104 shown by a1 to a4 inFIG. 18 are planes equivalent to the (100) plane (hereinafter,appropriately called {100} planes), or planes inclined from theseplanes. In a case of inclined planes, these are made to be planesinclined to a direction that the waveguide region spreads toward thestacked direction of the semiconductor layer. In the present embodiment,the following planes are used.

-   a1: (0-10) plane-   a2: (100) plane-   a3: (010) plane-   a4: (−100) plane

b1 and b2 in FIG. 18 are optical output faces, the (110) plane and the(−1-10) plane of the semiconductor layer structuring the waveguide areexposed. Mirrors are not formed on these planes.

The stacked structure of the semiconductor in the vicinity of the endfaces of the multimode waveguide 104 is the same as that described withreference to FIG. 15 and FIG. 16 in the fifth embodiment. Here, the corelayer 108 functions as a gain layer. Namely, abnormal growth of thesemiconductor layer is suppressed due to the planes equivalent to the(100) plane and the like being made to be the end faces, and the shapeshown in FIG. 4 is realized. In accordance therewith, current leakage isrestrained from being occurred, and optical loss is effectively reduced.

In the present embodiment, as a result that the side faces of themultimode waveguide 104 are made to be the specific planes as describedabove, those are made to have a shape whose corner portions areeliminated as compared with the conventional rectangular multimodewaveguide 104. The corner portions are regions which do not contributeto the emission intensity, and there is no need to flow an excesscurrent by eliminating the portions, and the advantage that an attemptcan be made to save the electric power of the device can be obtained.

The present invention has been described above based on the embodiments.The embodiments are exemplifications, and it will be understood by thoseskilled in the art that various modifications are possible, and suchmodifications are within a range of the present invention.

For example, in the above-described embodiments, the waveguides arestructured by using InGaAsP semiconductors. However, other group III-Vcompound semiconductors having zinc blend crystal structures may beused. For example, a group III-V compound semiconductor in which groupIII atoms include any one of B, Al, Ga, In, and Tl, and group V atomsinclude any one of N, P, As, Sb, and Bi may be used. Concretely,InGaAsP, AlGaInAs, AlGaInAsP, AlGaInP, InGaAsSb, InGaPSb, InGaAsN,AlGaInN, TlGaInAs, TlGaInAsN, TlGaInPN, or the like can be exemplified.Further, the example in which InP is used as a substrate has been shown.However, another semiconductors having a zinc blend crystal structuremay be used.

Further, in the above-described embodiments, those structured such thatthe end faces of the multimode waveguide do not include the {110} planeat all. Such a structure is preferable. However, those may be includethe {110} plane within a range which does not adversely affect on theelement performance. For example, ten percent or less of the side facesof the mesa structuring the multimode waveguide may be made to be the{110} planes. Here, it is preferably structured such that, among the endfaces of the multimode waveguide, the end face at the output face sideat which the emission port is provided does not include the {110} planeat all. In accordance therewith, optical loss and an extent ofreflection can be stably reduced.

Further, the couplers relating to the above-described embodiments arestructured such that the core layer is directly provided on thesubstrate. However, the lower guide layer is formed on the substrate,and the core layer may be provided thereon.

EXAMPLE

The optical amplifier having the structure shown in FIG. 17 and FIG. 18has been manufactured by the method described in the sixth embodiment.In the cross-sectional structure of the optical amplifier, pnp embeddingis preferably used in a forward direction device. The details are shownin FIG. 23 and FIG. 24. As shown in FIG. 23, the buried layer is astacked structure of a p-InP layer 114 and an n-InP layer 116.

The cross-sectional structure at a region G has been observed by ascanning microscope while changing θ₂ in FIG. 19. The results are shownin table 1. FIG. 25 is a diagram showing a state after the growth of themask in the process for obtaining the structure of FIG. 19. In thepresent example, the layered structure in this state has been evaluated.D and L shown in table 1 are the dimensions of the areas shown in FIG.25.

As shown in table 1, in a case of 45°, that is, the (100) plane, both ofvariations in building-up (D/d) and overhanging above the mask (L) canbe suppressed, and sufficiently little values with respect to thevariations have been obtained.

TABLE 1 MINIMUM D/d MINIMUM MAXIMUM VARIATIONS θ₂ (°) D/d VALUE OF DVARIATIONS VALUE OF L VALUE OF L OF L 0 1.8-2.2 1.8 0.4 0.5 0.6 0.1 301.4-1.8 1.4 0.4 2.1 3.2 1.1 45 1.2-1.4 1.2 0.1 0.5 0.6 0.1 60 1.4-1.71.4 0.3 2 3 1

It is apparently understood from the results of FIG. 1 that, when anangle of an end faces is made perpendicular to the optical waveguidedirection, namely, when θ₂ is set to 0° and the end face is the (110)plane, an extent (D/d) of abnormal growth of the end face is mademarkedly greater. Further, when θ₂ is set to a value greater than 0, andthe end face is provided so as to be inclined to the optical waveguidedirection, an extend of abnormal growth is reduced. However, it isunderstood that, when θ₂ is set to 30°, 45°, and 60°, the effect on areduction of abnormal growth is not sufficient, and when θ₂ is set to45°, namely, the end face is the (100) plane, D/d is markedly made less.

Further, it is apparently understood that variations in the values ofD/d are markedly made less when the end face is the (100) plane ascompared with the state in which another plane is used. This reason willbe inferred as follows.

FIG. 20 is an enlarged top view of the end face of the multimodewaveguide. When θ₂ in FIG. 19 is set to 45°, the end face of the regionG becomes the (100) plane, and the end face is structured from one flatatomic plane (a plane elongating horizontally from the left end to theright end in FIG. 20). However, when θ₂ is set to a value shifted from45°, the end face is structured from a concave-convex plane that planesequivalent to the (100) plane are connected with steps whose heights areat integer multiples of one atomic layer (the step-wise plane in FIG.20). Here, in the end face, the heights and the intervals (widths of theterraces) of the steps are uneven. In FIG. 20, variations of the stepsand the terraces in the in-plane direction of the substrate face areshown. However, the variations are caused in the same way in the <001>direction as well. It can be thought of that variations in a growth rateof the semiconductor layer in the vicinity of the end face are causeddue to such variations of the steps and the terraces, which varies anextent of building-up of the semiconductor layer.

Further, it can be thought of that, because the heights or the densitiesof the steps are made higher, and the terrace widths are made narroweras θ₂ is separated away from 45°, a growth rate more varies more easily,which causes an extent of overhanging above the mask to vary intensely.Concretely, when θ₂ is set to 45°, and the bonded surface is a (010)plane, a width of a terrace per one molecular layer step is madenarrower as θ₂ is made greater. When θ₂ is 5°, a width of a terrace ismade 11.4 times as high as a step, and when θ₂ is 7°, a width of aterrace is made 8.1 times as high as a step, and when θ₂ is 10°, a widthof a terrace is made 5.7 times as high as a step, and when θ₂ is 15°, awidth of a terrace is made 3.73 times as high as a step. Therefore, itcan be thought of that a growth rate varies more easily, which causes anextent of overhanging above the mask (building-up of the semiconductorlayer) to vary intensely. It can be understood from the abovedescriptions that building-up of the semiconductor layer can be stablysuppressed by providing an off angle less than or equal to 7° from the(010) plane.

1. A waveguide optical device comprising: a waveguide comprising a corelayer and a guide layer, wherein said core layer and said guide layerare formed from semiconductors having zinc blend crystal structures;wherein: said waveguide comprises fundamental mode waveguides and amultimode waveguide which has a width greater than a width of saidfundamental mode waveguides; said multimode waveguide includes sidefaces structured from one or more of: planes equivalent to a (100) planeof said semiconductors, planes forming an angle of inclination of 7° orless with planes equivalent to the (100) plane, in a direction parallelto a surface formed between said core layer and said guide layer; andplanes forming an angle of inclination with planes equivalent to the(100) plane, in a direction normal to the surface formed between saidcore layer and said guide layer; and said waveguide optical devicefurther comprises a semiconductor layer, wherein said multimodewaveguide is embedded as a buried layer within said semiconductor layer.2. The waveguide optical device according to claim 1, wherein thedirection normal to the surface formed between said core layer and saidguide layer is a <001> direction.
 3. The waveguide optical deviceaccording to claim 1, wherein said guide layer includes side faces whichare planes forming an angle of inclination of less than or equal to 5°with planes equivalent to the (100) plane, in a direction normal to thesurface formed between said core layer and said guide layer.
 4. Thewaveguide optical device according to claim 1, wherein said core layerincludes side faces which are planes forming an angle of inclination ofless than or equal to 5° with planes equivalent to the (100) plane, in adirection normal to the surface formed between the core layer and theguide layer.
 5. The waveguide optical device according to claim 1,wherein, at said side faces of said multimode waveguide, side faces ofsaid core layer are withdrawn from side faces of said guide layer. 6.The waveguide optical device according to claim 1, wherein said corelayer is formed from In_(x)Ga_(l-x)As_(y)P_(l-y), wherein x and y arenumbers greater than or equal to 0 and less than or equal to
 1. 7. Thewaveguide optical device according to claim 1, further comprising aplurality of input ports or a plurality of output ports, and a branchingfunction or a coupling function.
 8. The waveguide optical deviceaccording to claim 1, wherein said core layer includes a gain layer. 9.The waveguide optical device according to claim 1, wherein said corelayer includes a light receiving layer.
 10. A method of manufacturing awaveguide optical device, the method comprising: forming a stacked layeron a substrate, the stacked layer comprising a core layer and a guidelayer; forming a mesa portion, including fundamental mode waveguides anda multimode waveguide, by selectively removing said guide layer and saidcore layer; and forming a semiconductor layer, thus embedding aperiphery of said mesa portion therein, wherein side faces of saidmultimode waveguide form the periphery of said mesa portion; and whereinsaid side faces include one or more of: planes equivalent to a (100)plane of said stacked layer, planes forming an angle of inclination ofless than or equal to 7° with respect to planes equivalent to said (100)plane, in a direction parallel to a surface formed between said corelayer and said guide layer; and planes forming an angle of inclinationwith planes equivalent to the (100) plane, in a direction normal to thesurface formed between said core layer and said guide layer.
 11. Themethod of manufacturing a waveguide optical device according to claim10, wherein the forming said semiconductor layer comprises forming saidsemiconductor layer by epitaxial growth using a growth gas including ahalogen gas.